As display apparatus which include a cathode ray tube (hereinafter referred to as CRT), display apparatus of a high resolution and a high picture quality ready for, for example, a television broadcast called HDTV (High Definition Television) or a digital television broadcast have been and are being popularized.
That one of the apparatus mentioned above which is ready for the HDTV uses a horizontal synchronizing signal having a frequency equal to twice that of ordinary television receivers in order to achieve a high resolution. The frequency of the horizontal synchronizing signal mentioned is 31.5 KHz, for example, for the NTSC system. Meanwhile, it is prescribed that an apparatus ready for the digital television broadcast has a horizontal synchronizing signal having a frequency of 33.75 KHz for the NTSC system. Further, the high anode voltage to be supplied to the anode electrode of the CRT in such video apparatus as mentioned above is equal to or higher than 30 KV.
In the present situation, popularization of those cathode ray tube display apparatus having a raised resolution and having a screen of an increased size is increasing in this manner. Thus, for example, as television receivers, those apparatus have been popularized significantly which are designed so as to convert, for the NTSC system, the frequency of the horizontal synchronizing signal into a double rate mode of 31.5 KHz and besides can receive also an HDTV broadcast.
Therefore, where a high voltage direct current (dc) output voltage is applied to the anode electrode of the CRT in such a television receiver as just described, the high dc output voltage varies, for example, between the frequencies of 31.5 KHz and 33.75 KHz of the horizontal synchronizing signal. The variation of the high dc output voltage varies the luminance or the raster size of a screen displayed on the CRT. Therefore, it is essentially required to stabilize a power supply circuit for producing the anode voltage described above.
Based on the background of the foregoing situations, various switching circuits suitable for applications to various cathode ray tube display apparatus have been proposed.
An exemplary one of switching power supply circuits for video apparatus is shown in a circuit diagram of FIG. 6.
In the circuit shown in FIG. 6, a switching converter for receiving as an input and interrupting a rectified smoothed voltage Ei is of the self-excited voltage resonance type which includes a switching element Q1 and performs a switching operation. The switching converter is further formed as a switching converter of the complex resonance type wherein complex resonance is caused by a secondary side parallel resonance circuit and a primary side voltage resonance circuit of a converter transformer PIT (Power Isolation Transformer). Further, the primary side winding of a fly-back transformer FBT is connected in parallel to the primary side winding of the PIT and is driven for switching by the switching element Q1.
In the power supply circuit shown in FIG. 6, a voltage doubler rectification circuit for an commercial alternating current (ac) power supply AC is formed from rectifying diodes Di1 and i2 and smoothing capacitors Ci1 and Ci2. The voltage doubler rectification circuit produces a rectified smoothed voltage Ei equal to twice an ac input voltage VAC to the smoothing capacitors Ci1 and Ci2 connected in series and supplies the rectified smoothed voltage Ei to the primary side voltage resonance type converter. By forming the voltage doubler rectification circuit in this manner, a rectified smoothed voltage Ei of a sufficient level for a case wherein the ac input voltage AC is of the 100 V type can be obtained.
The power supply circuit further includes an active clamp circuit 20 for the secondary side of the power isolation transformer PIT. The active clamp circuit 20 includes an auxiliary switching element Q2, a clamp capacitor CCL and a clamp diode DD2.
To a first control circuit 1A in this instance, a secondary side dc output voltage E01 is inputted as a detection voltage. The first control circuit 1A applies a bias voltage, which varies in response to a variation of the level of the inputted secondary side dc output voltage E01, to the gate of the auxiliary switching element Q2. In response to the bias voltage applied, the amount of current flowing to the clamp capacitor CCL varies, and thereupon, the amount of charging current to a secondary side parallel resonance capacitor C2 varies. Consequently, also the level of the alternating voltage (parallel resonance voltage) obtained by the secondary side parallel resonance circuit varies. Stabilization of the dc output voltage to be obtained on the secondary side of the power isolation transformer PIT is achieved in this manner.
A high voltage generation circuit 40 indicated by a rectangle of an alternate long and short dash line is formed from a fly-back transformer FBT and a high voltage rectification circuit. The high voltage generation circuit 40 operates such that current induced in step-up windings NHV1 to NHV5 with excitation current flowing through a primary side winding N0 of the fly-back transformer FBT is rectified by five half-wave rectification circuits to charge a smoothing capacitor C0HV. As a result of the operation, a dc high voltage EHV of a level equal to approximately five times the induced voltage induced in each of the step-up windings NHV1 to NHV5 is obtained across the smoothing capacitor C0HV.
Further, a series circuit of resistors R1 and R2 is connected in parallel across the smoothing capacitor C0HV, and a voltage divided by the resistors R1 and R2 is inputted to a second control circuit 1B.
The second control circuit 1B outputs a control voltage, which corresponds, for example, to a voltage level variation of the dc high voltage EHV, as a control signal.
The level of control current to be supplied to a control winding NC is varied in response to the control voltage to variably control the inductance LB of a driving winding NB wound on an orthogonal control transformer PRT (Power Regulation Transformer). By the variable control of the inductance LB of the driving winding NB, the resonance condition of a resonance circuit of a self-excited oscillation driving circuit formed including the inductance LB of the driving winding NB varies. The variation of the resonance condition acts to vary the switching frequency of the switching element Q1, and this action fixes the dc output voltage to be outputted from the secondary side.
Incidentally, the circuit shown in FIG. 6 includes the power isolation transformer PIT and the fly-back transformer FBT, which are transformers of a large size, and requires a considerably large mounting area. Therefore, it is demanded to miniaturize the circuit.